A technique for welding metal materials by friction stir welding (FSW) is known among the conventional methods for processing metal materials. In friction stir welding, metal materials that are to be joined are disposed opposite each other in a joining portion, a probe provided at the distal end of a rotary tool is inserted into the joining portion, and the two metal materials are jointed together, while rotating the rotary tool, along the longitudinal direction of the joining portion. The friction stir welding makes it possible to obtain a good joint strength, but such welding is difficult to apply when the joining portion has a curved surface and when the rotary tool is difficult to insert. For this reason, when a structure such as a rail car body is manufactured by friction stir welding, the metal materials are processed by a combination of friction stir welding and a fusion processing such as MIG welding, rather than by the friction stir welding alone, and a structure having a curved surfaces is manufactured (for example, see the Description of U.S. Pat. No. 3,224,092).    [Patent Document 1] JP Patent No. 3224092.